It is known to provide an apparatus for the treatment of materials upon horizontal surfaces whereby the treatment involves, at least in part, the displacement of the material by the scraping and wiping action of one or more blades drawn over the surfaces by one or more arms.
For example, the apparatus can be a rotary dryer of the plate type in which a plurality of vertically spaced annular plates have upper surfaces receiving the material to be dried, a heating or cooling fluid being introduced into the plates. A shaft or rotor surrounded by these plates has arms extending radially above each plate level and entraining scraper blades which are intended to displace the solid material on each plate inwardly or outwardly so that this material can pass onto the next-lower plate for treatment on the plates in cascade.
In such an apparatus, the arms each carry a plurality of blade holders which are disposed axially in succession on the arm and which can pivot about the axis of the arm to allow the holders to swing upwardly or downwardly. The holders support the blades at angles to the radial direction and to the axis of the arm so that, as viewed along a perpendicular to the arm in the plane of rotation thereof, the blades on each arm appear to overlap, the specific orientation as the blades are drawn along the surface being such that the material is urged either inwardly toward the inner periphery of the annular plate to fall onto the inner periphery of the next-lower plate, or outwardly so that the material can fall onto the outer periphery of the next-lower plate.
To this end, the inner and outer peripheries of the successive plates are staggered and the forward or working face, i.e. the surface of the blade engaging the material, includes an angle of somewhat more than 90.degree. to with the plate or treatment surface.
Systems of this type are described or illustrated, for example, in the commonly assigned U.S. Pat. No. 3,460,269. The arm and blade constructions which can be used in the plate dryer diagrammatically illustrated in U.S. Pat. No. 3,460,269 can be found in Tellertrockner und Tellerkuhler (Plate Dryer and Plate Cooler) published by our assignee Krauss-Maffel AG of Munich, Germany, and identified as Publication No. B4702.
Reference may also be had to German Pat. No. 476,835 and German Pat. No. 100.964 showing rotary assemblies of this general type.
The rotary blade shaft extends centrally through the plate stack and is perpendicular to the treatment surfaces of the plates which may be circular and can lie in horizontal planes as has previously been noted. There are various arm and blade arrangements which have been employed; typically a plurality (for example four) of arms extend radially outwardly from the shaft above each surface in angularly equispaced relationship with each blade mounted on the respective arm by a holder which has eyes traversed by the arm and projects transversely thereto, the free end of each holder carrying a respective blade and being immovably fixed thereto. The holders of the adjacent blades directly abut against one another and are threaded onto the arm.
Because of the mutually abutting relationship of the row of holders, they are fixed in the longitudinal direction, i.e. along the axis of the arm. The blades themselves are stamped from sheet metal, can be bent and have edges juxtaposed with the treatment surface. The blades are oriented so that they overlap as seen perpendicularly to the arm and thus form an uninterrupted sweeping surface.
The two spaced-apart eyes of the holder have holes which are stamped in the material from which the holder is made, generally sheet metal, these holes generally being vertically elongated or slot-shaped to compensate for the various tolerances in constructing the plate-type treating unit, especially variations in distance between the axis of the arm and the circular suface. As a result, the blades hang downwardly toward the surface and are dragged across the latter by the arm.
Experience has shown that such arrangements have certain disadvantages. For example, in spite of the presence of the elongated holes, variations in the distance between the arm and the circular treatment surface because of manufacturing and assembly tolerances and because of thermal expansion and contraction phenomena may not be sufficiently compensated and can result in different or varying friction forces between the lower edge of the blade and the upper surface of the circular plate and between the forward or working surface of the blades and the material engaged thereby.
As a result, the blades tend to become canted so that only a corner actually scrapes upon the circular treatment surface and the pattern of engagement of the material on the surface is a saw tooth, thereby reducing transport of the material, interfering with the uniformity of treatment, reducing the output of the apparatus and resulting in nonhomogeneous treatment.
In addition, because the weight of the blade and its holder may bear upon the plate surface only at a corner of the blade, there is an increase in blade wear and possible damage to that surface. In fact, experience has shown that the nonuniform engagement of the blades with the material can result in bending of the arm or of the holder.
When the treatment surface is heated, moreover, the heat transfer to the material may be poor over most of this surface because of nondisplacement of the material and the development of solid and coherent layers (caking) upon this surface.